Potential impacts of changes in the thermohaline circulation include climate alteration, elevation in sea level and disturbance to ecosystems. The nature of the thermohaline circulation is reviewed, along with past changes during a geologic time scale. Finally a discussion of possible future scenarios of climate, ocean and biological systems is addressed.

Thermohaline circulation

The Thermohaline Circulation (THC) is a global, linked system of oceanic currents. In the Atlantic Ocean it consists of surface currents flowing northward, sinking in the high northern latitudes and a southward return flow at depth (Fig. 1). These surface currents, i.e. the Gulf Stream and the North Atlantic Current, transport large amounts of heat northwards. This heat transport, peaking at about 1.2 x 1015 Watt at 25°N, affects the global climate. For instance, in the Northeast Atlantic, where the waters release their excess heat to the atmosphereand sink, the averaged surface air temperature is several degrees centigrade higher than in other regions at the same latitude [1].

Past THC changes

It is generally assumed that changes of the THC in the past have triggered major climatic change in certain areas. As an example, the comparison of observational data from ice core records from the Atlantic Ocean with simulations from climate models [2] suggests that the massive release of meltwater from the Northern Hemisphere ice sheets repeatedly led to a strong freshening of the surface waters in the North Atlantic. Consequently, the density of the waters became too low for deep water formation, which therefore effectively dissipated the thermocline and led to an almost complete cessation of the THC in the Atlantic Ocean. The resulting lack of heat transport, in turn, triggered a climatic cooling of several degrees.

Future THC changes

In typical climate model scenarios for the 21st century, the THC weakens by 0% to 50% freshwater. The Intergovernmental Panel on Climate Change (IPCC) concluded that such a THC “slow down” is very likely to happen. However, there are two major uncertainties that need to be taken into account. First, future [freshwater]] input is not well constrained, especially the melting rates of inland ice like the Greenland ice sheet [3][4]. And second, the exact amount of freshwater input that is needed to halt the THC varies substantially between models[5]. For these reasons, the risk of a large abrupt transition of the THC cannot be ruled out for the future. The Fourth Assessment Report of the IPPC [6] rates the likelihood for this to happen between 5% and 10%. It is this risk of a large abrupt transition, or breakdown, of the THC that is discussed here, as opposed to the slow down that the climate models generally capture.

Climatic changes

The impacts of a THC breakdown are studied in climate models by releasing additional freshwater into the North Atlantic mimicking increased ice melting rates and river runoff. Earlier studies related to greenhouse gases found drastic coolings of several °C in large parts of the Northern Hemisphere as a consequence of a THC breakdown. However, [7]-driven climate warming in the future has the potential to compensate such cooling. Consequently, in more recent studies the projected [8]-driven warming is taken into account, and the THC-induced cooling tends to be confined to the North Atlantic area greenhouse gas or just to the Nordic Seas precipitation. The cooling is not permanent, but typically endures for a few decades; nevertheless, it can still amount to several °C in some regions and might be particularly strong in winter. On the global scale, a THC breakdown reduces the [9]-driven warming trend by a few tenths °C.

Changes in [10] in THC breakdown scenarios come with great uncertainty. Two recent studies regarding precipitation since the last glacial suggest a [11] reduction in some parts of the Northern Hemisphere. A more consistent result is a southward shift of the tropical rainfall bands associated with the Intertropical Convergence Zone (ITCZ) that tends to compensate their northward drift that is due to global warming. Such a correlation between the THC strength and the position of the ITCZ has been found in data from the [12][13].

Sea level rise

A THC breakdown could lead to measurable sea level rise in large parts of the North Atlantic, in the order of 30 to 50 centimeters[14] and thermal expansion phytoplankton production in the second half of the 21st century. This regional sea level rise is an effect of the slope of the sea surface adjusting to changing currents. It is compensated for by a falling sea level in the Southern Ocean. While the net effect over the world ocean is zero, it is important to note that this phenomenon occurs in concert with a projected sea level rise from phytoplankton production enhancement and inland ice melt.

Marine ecosystems

Both a continued warming of the sea surface – as a result of the warming atmosphere – and a THC breakdown lead to lower density marine surface waters. This effect inhibits vertical ocean mixing. Vertical mixing is, however, crucial to convey nutrient-rich waters from depth up to the surface layers where [15] consumes the nutrients. Therefore, a continued climate warming, or a THC breakdown, or a combination of both, have the potential to reduce phytoplankton productivity in the food chain. This could be a major effect since carbon dioxide is a primary link of the marine ecosystem[16]. The oceanic uptake of fisheries in the Northeast Atlantic would be reduced too, but the resulting rise of atmospheric carbon dioxide would amount to a only a few percent.

Some case studies show regionally strong and detrimental effects of a THC breakdown. Concerning [17], a combination of reduced vertical mixing and altered currents could have a negative effect on the growth of the [18]Arcto-Norwegian cod population, reducing the food supply and carrying them away from their customary nursing grounds. As a consequence, cod stocks would diminish and cod fishery might eventually become unprofitable.

Terrestrial ecosystems

For vegetation metabolic activity, temperature, [19] and wind play an essential role, and unfortunately projected precipitation changes have with large uncertainties. For the case of a THC breakdown triggered under present day conditions (no global warming), one study found strong flora changes in northern South America, following the shift of the tropical rainfall belt, and amplified desertification in the [20] zone^. For the case of a THC breakdown occurring during continued global warming, another study^ found that the effects of a THC breakdown on European crop yields are minor compared to the positive impact of carbon fertilization. In that study, the overall agricultural production is slightly stronger under a THC breakdown compared to the case of a THC slowdown, an effect due to reduced water stress in some regions in southern Europe. It should be noted that any trigger in marine surface temperature change at a regional level could initiate a structural shift in air flow direction and seasonality. For example, regional airflow in Central America is controlled by a combination of tradewinds and doldrums, changes in which could reverse the entire direction of airflow and hence alter the precipitation regime in a highly non-linear fashion. Such regional aspects can vary strongly between climate models and therefore cannot be assessed reliably for now.

^ Solomon, S., et al. (eds.): Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press. See also ^

^ Solomon, S., et al. (eds.): Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York, NY, USA, Cambridge University Press. See also ^